The objective of the proposed research is to correlate measurements of atrioventricular node (A-H) and infranodal (H-Q) conduction times and refractory periods with total mortality, incidence of sudden death and progression to higher grades of AV block in patients with bundle branch block (BBB). His bundle recordings will be obtained in patients with bundle branch agreeing to enter this study in order to estimate AV nodal and infranodal conduction time. These patients will be followed either in the cardiac clinic at San Francisco General Hospital, University of California, San Francisco or by private referring physicians. Either the patient or referring physician is contacted at 3 month intervals and pertinent physical findings, symptoms and a 12-lead ECG are recorded. A 24- hour continuous ECG monitor is obtained at least 3 times per year and more often when clinically indicated. In the event of death, a Public Health Nurse interviews the patient's family or friends, reviews the death certificate or results of the postmortem examination (where available) in order to determine the cause and suddenness of the demise. Documented episodes of 2 degrees and 3 degrees AV block will be compiled. Analyses of the data will hopefully allow us to define a subset of patients with BBB who are at increase risk for development of either sudden death/or high grade AV block and may have important implications with regard to prophylactic cardiac pacemaker implantation in patients with BBB.